At this point, packages for -release(which includes -stable...) are only updated at each new release. There was a time when some-stable(& rarely -release...) packages were updated mid-cycle along with some security updates trickling down, but resources are currently such that there isn't enough manpower involved with the project to continue this practice. Perhaps in time it will resume.

There is no difference between the packages provided by the project to what can be compiled via a local ports tree. The project recommends that all OpenBSD users use packages to save both time & bandwidth to the mirrors. For more information, study Section 15.1 of the FAQ: